Conventional photoconductive detectors comprise one or more square elements of photosensitive material, each element having a pair of spaced bias contacts. For imaging applications, such a detector is placed in the image plane of an optical assembly and is usually shielded to reduce the incidence of background illumination upon the detector. The detector is usually mounted on a cold stage and is cooled to enhance signal-over-noise discrimination. In one form of conventional detector apparatus using intrinsic photoconductive elements responsive to the middle and far infra-red region of the spectrum, a steady direct current (DC) bias, from a constant current source, is applied to each element. There is thus developed across each detector element a bias pedestal voltage, a voltage dependent on bias current magnitude and element resistance. When radiation of appropriate wavelength is incident upon the detector elements, photosignals--in this case photo-voltages--are developed and these increment the voltage provided by each element. The incremental photosignal voltage is, for normal radiation intensities, of magnitude several orders smaller than the magnitude of the bias pedestal, and it is usual to back-off each element voltage by subtracting DC voltage to allow extraction and amplification of the photosignal component. However, to be wholly effective the back-off voltage applied, in each case, must follow changes in the pedestal voltage. Such changes may occur, for example, as a result of cold stage temperature drift, of change in ambient temperature, of change of average background illumination, and of current drift. Such pedestal voltage changes are in general also orders of magnitude higher than the photosignal increment. Furthermore the pedestal voltage and the change of this voltage will vary from element to element. In general the resistance of each element will differ, since material resistivity and element dimensions vary within manufacturing tolerance. Because of non-uniformities in the bias pedestal, it is in the very least difficult, if not impractical, in unscanned, so-called "staring" systems, to back off element voltage satisfactorily so that the wanted illumination dependent photosignal can be extracted without the introduction of an unacceptable degree of fixed pattern noise. It is also possible to operate these detectors using constant voltage drive bias instead of constant current in which case device current is measured. This too requires bias compensation, and this likewise introduces fixed pattern noise.
Because of these difficulties, progress in photoconductive detector development is impeded and this development is giving way to the alternative development of photovoltaic detectors; albeit this latter involves a more complex, generally more expensive and less far advanced technology.